This invention relates to the production of inorganic silicate materials and, more particularly, to a method of thermally expanding silicate-based compositions to produce lightweight, cellular aggregate processing advantageous acoustic and thermal insulating and fireproofing properties.
The production of expanded silicate aggregate by thermal expansion is well known. U.S. Pat. No. 3,765,919 discloses a process of producing a silicate material capable of being expanded at a construction site, or as part of the manufacturing process, by the application of heat. This is achieved in an "expander-dryer" in the form of a rotary calciner in which heated air at elevated temperatures, between 880.degree. and 1,020.degree. F. is utilized. A preexpansion can be achieved in an "oven-dryer" at temperatures of 350.degree. to 400.degree. F. U.S. Pat. No. 3,719,510 discloses a process of preparing an expanded insoluble silicate aggregate in which a silicate composition is ground, after mixing and curing, to a particulate form and subsequently expanded by exposure to temperatures of 800.degree. to 1,800.degree. F. to obtain rapid and complete expansion.
U.S. Pat. No. 4,203,773 states that the processes described in U.S. Pat. Nos. 3,719,510 and 3,765,919 involve several steps which are difficult to control and discloses an alleged improved method for preparing waterinsoluble expanded silicate aggregate in which particles are thermally expanded by rapid heating to at least 425.degree. C. and generally in an effective range of 425.degree.-750.degree. C.
U.S. Pat No. 2,553,759 discloses a method for making porous refractory bodies by coating combustible globules with finely divided refractory material, drying the same on a vibrating table by the use of suitable heating means such as infrared lamps and then firing the dried globules under oxidizing conditions at a temperature sufficient to burn out the combustible material from the interior and cause the desired sintering. U.S. Pat. No. 3,743,601 discloses a process for forming microcellular, inorganic silicate foams by expanding hydrated particulate silicates by thermal energy supplied by a source of conventional heat, microwave energy, dissipation of mechanical energy and/or an exothermic reagent to provide part or all the heat input. Alternatively, the hydrated mass can be uniformly heated under sufficient pressure to prevent loss by evaporation of the water of hydration which, upon rapid release of the pressure, will act as a blowing agent and expand the mass. When microwave energy is utilized, it is applied at a frequency of 2450 megacycles. U.S. Pat. No. 3,756,839 also discloses the production of alkali metal silicate foam involving the rapid input of heat energy which, the patent states, may be supplied by commercially available heating means such as a radiant heat oven, a convection oven or, in particular, a microwave oven. In a conventional convection or radiant heat oven the material is heated to 500.degree. C. for one hour.
U.S. Pat. No. 3,830,892 discloses a method for manufacturing a molded article of expanded vermiculite which comprises subjecting vermiculite to heating or radiation with electromagnetic waves in the presence of urea or thiourea and, during or after the process, allowing the vermiculite to contact with an aqueous solution of ammonium or alkali silicate. The patent states that vermiculite is generally expanded at temperatures beyond a critical level of 800.degree. C. The treated vermiculite is expanded, according to the patent process, by heating at a temperature of 160.degree. to about 500.degree. C., desirably about 220.degree. C., for about 10 minutes to one hour or by radiating the vermiculite with electromagnetic waves. The electromagnetic waves employed are said to be usually microwave or far infrared radiation in which instance the duration of radiation ranges from fractions of a second to thirty minutes. The electromagnetic waves may be supplied by any known sources such as an electronic range or high frequency welder.
Thus, it can be seen that the prior heat processes used to cause expansion of inorganic silicate particles are relatively energy intensive. Thus, such processes generally require heating the particles to extremely high temperatures to produce an expanded particle of acceptable density. The processes used for expansion may vary somewhat but generally depend on heating air to high temperatures, 1000.degree. F. or more, and contacting the particles with this air at high velocity or utilize rotating pipes heated to temperatures in excess of the reaction temperatures of expanders and allowing the particles to contact the surface of the pipes during rotation until sufficiently expanded. Since the particles to be expanded are generally considered to be thermal insulators, as well as to some extent refractory, rapid thermal expansion requires a sufficient amount of heat to overcome the inherent insulating properties of the particles so that uniform expansion to the proper density may occur in as short a time as possible. Alternatively, at lower temperatures long residence times have generally been required to achieve the desired expansion. Consequently, the efficiency of the expansion processes heretofore disclosed possess disadvantages for practical commercial application in which the most economical use of heat energy is desirable.